Hello~I hope your son is still well and not having any reaction to the varnish. Being he did it outside, I doubt that there is as much concern however than if it were inside. That being said, the next time he does it, just make sure he wears protective clothing and the mask.
Outside is much less of a problem for anything. Have you checked the canister or can it came in? Some are made to be non toxic. Now, paint fumes or toxic fumes can lead to headaches, confusion, dizziness, etc. If he has anything like that, call his doctor asap or get medical care for him. But check the bottle/brand or provide that info here. That would help. And don't forget poison control! They have a 1800 number and are extremely helpful. I've had to call them myself.